1. Field of the Invention
This invention relates to in-flight calibration of aircraft pitot-static systems. More specifically, the invention models pressure error as a continuous function of airspeed rather than computing error for discrete airspeeds. High data-rate measurements of static and differential pressure and Global Positioning System (GPS)-based ground speed measurements are utilized for computing pressure errors over a range of airspeed.
2. Description of the Related Art
Pressure-based airspeed and altitude systems are susceptible to errors in measurements of static and differential pressures. Static pressure errors are typically introduced by the disturbances in the How field around the aircraft, which necessitates careful positioning of static pressure ports to minimize this effect. Errors in differential pressure sensing from a pilot tube can be caused by excessive flow angularity or flow field interferences with the aircraft. Because of these potential errors, pressure-based airspeed and altitude measurements for aircraft typically require calibration of the installed pitot-static system. Several methods and devices have been used for in-flight pitot-static calibration. These include tower fly-by, pacer aircraft, and trailing cone methods.
The approach to in-flight calibration methods generally involves comparison of onboard airspeed and altitude measurements with “truth data” such as ground referenced speed and altitude or measurements from a calibrated aircraft. A common practice for pitot-static system calibration is to assume all pressure errors are due to static pressure measurements that in turn are used to derive airspeed corrections.
The introduction of satellite-based positioning systems enabled new in-flight calibration methods based on accurate ground speed measurements. Generally, these techniques involve flying a defined flight track, such as a triangle or square, at constant airspeed and heading and solving for the wind speed, wind direction and true airspeed. Calibrated impact pressure (qc) is then compared to the measured impact (or differential) pressure (qci) to compute the error in terms of static pressure and/or calibrated airspeed. Pressure errors are often presented in the form of normalized pressure error (Δp/qc) versus measured differential pressure (qci). This approach requires completion of multiple flight patterns for each airspeed and configuration, which can require lengthy flight time and associated costs. These systems are not practical for in-flight calibration of pitot-static systems for remotely piloted, dynamically-scaled aircraft due to confined test range size and limited flight time available for calibration flights.